1. Field of the Invention
This invention relates to a process for and an apparatus for removing at leaast one acidic component from a gas by treating waste gas containing the at least one acidic component such as hydrogen chloride, hydrogen fluoride, nitric acid, hydrogen sulfide, or sulfurous acid. Waste gases containing al least one acidic component are produced in various chemical industries, semiconductor industries and others.
2. Prior Art
It is conventinal to carry out a process for removing at least one acidic component from a waste gas by washing the gas with an alkali-containg solution to absorb the acidic component and neutralize the acid with alkali in order to remove the acidic component from the gas.
Different devices for carrying out an absorption operation involving gas-liquid contact reactions are also known prior to this invention. A packed column having various kinds of packing materials, a spray column, a scrubber, a cyclone scrubber, a wetted wall column, a plate column, a bubble column, a bubble-agitating column and the like are chosen and used independently or in combinaton in accordance with conditions which are required.
When the acidic gas such as hydrogen chloride or nitric acid is contained in the waste gas and the gas is discharged in the atmosphere, a white fume occurs due to an acid mist even if the coefficient of removal of the acidic gas reaches 70-90%. Therefore, even if the value of the environmental or hygienic standard is achieved numerically, the occurance of a white fume is not always sufficient to satisfy all people. A technique for removing the acidic components from such waste gas to a substantially zero level of content has been searched for. However, it has been difficult to develop means to meet all of the requirements on economic basis, to enable continuous operation over a long term and to practice the process in a small space.
When washing a gas containing acidic component(s) by using an alkali-containing aqueous solution of relatively high concentration in some kinds of absorption columns, absorption and removal of the acidic component(s) may be effectively carried out. However, a salt such as an alkali carbonate is produced by reaction between carbon dioxide in the gas and the alkali and then deposited or precipitated and piled up in the column thereby making operation impossible at all. Therefore, such process not only requires a periodical complicated washing operation but also provides a disadvantage of discharging the alkali-containing aqueous solution as waste during the washhing operation, even though the alkali-containing aqueous solution has a fairly high concentration.
A process for washing an acid gas-containing gas with a low concentration of an alkali-containing aqueous solution was tried, but the process was not sufficient to remove the acid gas by using the alkali-containing aqueous solution of low concentration to an extent such as not to precipitate alkali carbonate crystals. A white fume was apt to occur when the processed gas is discharged into the atmosphere. Extension of residence time of the waste gas has been tried in order to prevent occurance of such a white fume. It was necessary to enlarge the absorption column or to connect a number of columns in series to make a multistage system. As a consequence, the overall size of the apparatus becomes larger and pressure loss increases thereby causing an increase of running cost.